We propose to continue research in the general area of spontaneous and induced variation and mutation in somatic cells in culture. The cells used, especially hamster fibroblasts, human lymphoblasts, and human fibroblasts, range from frankly heteroploid to truly diploid. Our objecties are: 1. Investigate further the molecular nature of the unique variants in heteroploid hamster lines which overproduce dihydrofolate reductase, concentrating on the isolation of specific messenger RNA; 2. Isolate variants deficient in dihydrofolate reductase; 3. Pursue the possibilities of orderly transcription and of specific mutagenesis in synchronized cultures of lymphoblasts; 4. Attempt to produce and characterize non-cycling cells in lymphoblast cultures; 5. Isolate lymphoblast variants heterozygous at an autosomal drug-resistant locus or homozygous at an autosomal auxotrophic locus; 6. Further purify and characterize hypoxanthine-guanine phosphoribosyltransferase from red blood cells, lymphoblasts, and fibroblasts; 7. Investigate the infection of fibroblasts with the SV40 virus, in regard to differences in virus production and the susceptibility of cells from certain individuals; 8. Initiate studies concerning the possibility of gene amplification in dihydrofolate reductase-overproducing variants and the cloning in vitro of the relevant gene.